Such purifiers comprising a solids separation device are known from the prior art. An example purifier is disclosed in EP 2 065 344 A1. Upflow anaerobic sludge blanket (UASB) wastewater treatment is a process that generally employs a reactor having a reaction vessel that comprises a bed of granular anaerobic sludge. Influent wastewater is evenly distributed beneath the bed and flows upward through the biomass bed. The fluid is continuously in motion in the reaction vessel due to gas flows that find their way upwards through the fluid towards the liquid level.
In the known embodiment, near the top of the reaction vessel, the water phase is separated from sludge solids and gas in a three-phase separator (also known as a gas-liquid-solids separator) or solids separation device. EP 2 065 344 A1 discloses a solids and gas separation device comprising a solids collection chamber. Fluid from the process or reaction chamber can enter the separation device and the solids collection chamber. The separation efficiency is higher in conditions of a relatively low turbulence. Alternative solids separation devices, not necessarily comprising a solids collection chamber, are known, such as whirlpool devices and sieves.
In the known embodiment, separated fluid with solids from the solids separation device are transported back to the reactor, where due to density differences a downward flow is initiated. In the lower part of the reactor the fluid will mix with the sludge or with influent. Gas bubbles, for example formed in the anaerobic processes, in the fluid cause an upward transport. In this manner, a circulation of fluid flows is established inside the reaction vessel.
A drawback of a known purifier with a solids separation device is that the fluid that is being separated in the solids separation device comprises gaseous components, or bubbles, that rise upwards and create turbulence. These bubbles can also give the heavier sludge particles an unwanted upward lift, which interferes with the separation process in the solids separation device. This effect is known as “sludge flotation” or simply “flotation”. These drawbacks limit the efficiency of a solids separation device to separate sludge from other liquids and gases, and thus limit the functioning of a purifier.
According to the prior art the sludge bed in the reactor should be sufficiently far below the solids separation device so that the separated particle matter in the solids collection chamber can drop down through the liquid into the sludge bed. If the sludge bed effectively surrounds the solids separation device, this dropping down effect would not occur, and the solids separation device would fill up with particulate matter and stop functioning. As such, known solids separation devices place specific demands on the level of the liquid and the sludge bed during operation, reducing the freedom of the designer of a purifier.
Installing the solids separation device near the top of the reaction vessel is a further drawback, complicating the construction of the reactor.
EP 0 493 727 discloses a combination UASB and mechanical purifier having an external whirlpool separation device connected to an external pipe. Internally, the purifier reactor bottom forms a mechanical separation device that allows particulate matter to settle in a settling chamber underneath the mechanical separation device. The particulate matter cannot traverse the mechanical separation device in the opposite, upward, direction. A drawback of this system is that two distinct types of separation devices are used, and that the external separation device requires pipes going through the reactor vessel. Moreover, sludge will settle in the settling space below the influent lines where the sludge no longer interacts with the influent material, so that the efficiency of the anaerobic processes is not optimal